“EVERYTHING in the cosmic universe is composed of matter and form. Everything is concrete and individual. Hence the forms of cosmic entities must also be concrete and individual. Now, the process of knowledge is immediately concerned with the separation of form from matter, since a thing is known precisely because its form is received in the knower. But, whatever is received is in the recipient according to the mode of being that the recipient possesses. If, then, the senses are material powers, they receive the forms of objects in a material manner; and if the intellect is an immaterial power, it receives the forms of objects in an immaterial manner. This means that in the case of sense knowledge, the form is still encompassed with the concrete characters which make it particular; and that, in the case of intellectual knowledge, the form is disengaged from all such characters. To understand is to free form completely from matter.
“Moreover, if the proper knowledge of the senses is of accidents, through forms that are individualized, the proper knowledge of intellect is of essences, through forms that are universalized. Intellectual knowledge is analogous to sense knowledge inasmuch as it demands the reception of the form of the thing which is known. But it differs from sense knowledge so far forth as it consists in the apprehension of things, not in their individuality, but in their universality.
“The separation of form from matter requires two stages if the idea is to be elaborated: first, the sensitive stage, wherein the external and internal senses operate upon the material object, accepting its form without matter, but not without the appendages of matter; second the intellectual stage, wherein agent intellect operates upon the phantasmal datum, divesting the form of every character that marks and indentifies it as a particular something.
“Abstraction, which is the proper task of active intellect, is essentially a liberating function in which the essence of the sensible object, potentially understandable as it lies beneath its accidents, is liberated from the elements that individualize it and is thus made actually understandable. The product of abstraction is a species of an intelligible order. Now possible intellect is supplied with an adequate stimulus to which it responds by producing a concept.”
~From Thomistic Psychology: A Philosophical Analysis of the Nature of Man, by Robert E. Brennan, O.P.; Macmillan Co., 1941. (Additional paragraphing and emphasis added).